A simple way to prepare calcium oxide from the eggshells or mushels

An eggshell consists mainly of calcium carbonate, which makes up to 97% of the total eggshell mass. The rest are the proteins, which form a network stabilizing the structure of an eggshell. An average eggshell weighs appr. 6 g, the thickness of it varies between 240 and 420 mkm.

Because the eggshell is so thin, it can be relatively easily heated above 800°C — the temperatures, at which the decomposition of CaCO3 occurs with a reasonably fast rate. Here I propose a rapid and a simple experiment demonstrating the calcination (decomposition) of the eggshell calcium carbonate into calcium oxide (reaction scheme follows).

Experimental procedure

The shells of chicken eggs were thoroughly cleaned, the membrane being mechanically removed off the inner side of the shell. 

Attention! Wear safety goggles when you heat eggshells, since they tend to crack in the flame and little pieces may suddenly jump off and injure your eyes.

A piece of an eggshell about 4–9 cm^2 was held by tweezers and its surface was burned with a kitchen blow torch (Atomic; supplied by the local grocery store). The heating on the same spot was continued until it started to glow and several seconds thereafter to ensure the complete combustion of the protein network of the eggshell. The flame was applied uniformly throughout the free surface of an eggshell piece, where it was not covered with the tweezers. The completion of the decomposition was achieved, when the desired area has acquired a rough surface and changed its color into ash-grey (picture).

Some physico-chemical properties of CaO

Calcium oxide was allowed to cool down. Few droplets of water were subsequently added. Calcium oxide realeases heat on the reaction with water.

To the obtained calcium hydroxide suspension a few droplets of phenolphthalein were added. The appearance of the pink color  confirms the formation of Ca(OH)2.

Conclusion

Eggshell is a readily and easily available material at home. Among the educational or amator projects involving eggshells it is worth to mention the conversion of eggshell calcium carbonate into its acetate and the subsequent dry distillation of it into acetone [ref]. In the context of the demonstration experiments the decomposion of eggshell calcium carbonate seem to be unexplored.

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